RcTek : Information & Resources for the Model Car Racer

Model Car Differentials - How They Work

This section of the RcTek site is devoted to differentials, what they are, what they do and the different types of that can be fitted to radio controlled model cars.

This particular article aims to explain how a basic or, as it is sometimes referred to, open differential actually works.

Open Differentials

An open differential is so called basically because there is no form of limitation upon the differential action, it remains unlocked whether there are changes in the available traction or there are speed differences between either of the output axles.

This points made in this article do not take into consideration an open diff that is fitted with seals and is filled with an oil, this we would interpret as a limited slip differential.

There are a many types of differentials available for use in radio controlled model cars, but they all are based around overcoming the limitations of the open differential, so the reading of this article is recommended whatever kind of differential you have in your model car.

The differential used as an example in this article is of a typical design used in model car racing and some parts have been left out or simplified for the sake of clarity.

All the parts of the differential are shown in our Open Differential Parts Identification article, which you should read to familiarise yourself with the part names used in the text below.

Important Notes

Model car differential gears

As the action of the differential occurs by gears rotating inside a casing that is itself rotating, we have tried to simplify the representations of the actions. This requires you to firstly understand a few things by studying the image to the left, which shows the parts of the differential with the casing removed; The output cups and the sun gears are shown in green as they act as one unit, but the left and right hand sides can act independently. The ring gear and pivot pin are ordinarily fastened to the differential casings an also act as one unit. The planet gears are free to rotate on the pivot pin.

Differential Gear Side View Animation

Playing the differential gear side view animation on the right primarily shows a side view of how the pivot pin rotates the planet gears around inside the differential.

Also shown are the sun gears, planet gears and pivot pin to act as a colour coded guide for identifying the parts and where they are positioned within the differential.

Understanding how these parts move in relation to each other is crucial to interpreting the rest of information and animations in this article as the pivot pin would be rotating end over end away from you if you were looking at a real differential. You may Stop the animation if required.

In the following animating the non-moving red arrow represents the direction of travel of the model car and the rotational movement of the gears is represented by the small blue arrows.

Effects of the Sun & Planet Gear Arrangement

The arrangement of the sun and planet gears in an open differential creates to a situation where no matter from which input or output the rotation is applied there will always be rotation of at least one other shaft.

Reverse Action Animation

Which and how many of the shafts rotate is entirely dependant on the load present on one or two of the other shafts.

This is is even true if there is no input rotation and this is demonstrated when you Play the Reverse Action Animation shown to the right. You may Stop the animation if required.

The animation shows the result that you would get if you were to stop the differential casing from rotating whilst turning one of the output shafts with the wheels not touching the ground.

When the output shaft is turned, the pivot pin, being attached to the casing, does not itself rotate as in the Differential Gear Side View Animation above left. Forward rotational movement of the left hand output will be transferred via the planet gears (as they are free to rotate around the pivot pin) to turn the right hand output in a reverse direction.

Whilst this may explain why you have seen this action occur it may also seem irrelevant to the process of creating a difference of speed between the driven wheels on the car.

What it demonstrates is the ability of the differential to allow the wheels to travel at different speeds (even if they are in apparently opposite directions). But, if you look at this from the point of view that where a right hand wheel is going faster than the left hand, the left hand one could be thought of as going backwards in relation to the right one.

Confused? - See the information below.

Forward Motion Animation

Reverse Motion Animation

If you Play the forward motion animation the moving yellow car overtakes the motionless green car. You may Stop the animation if required.

When Played, this reverse motion animation shows a motionless yellow car that the green car reverses past. You may Stop the animation if required.

The point these animations are intended to make is that the net result of both is that the yellow car has effectively travelled forwards a greater distance than the green one.

If we relate this to the reverse action of the differential above we hopefully show the potential an open differential has for creating a speed difference between the wheels.

Equal Allocation of Torque

An open differential will always try to allocate the torque equally, which may initially sound good, but this is where it fails as far as providing equal traction is concerned when out of balance forces exist.

Straight Line Action of an Open Differential

When a model car is travelling in a straight line on a smooth surface, the weight on both the left and right hand wheels is, for all intents and purposes, equal.

As both sun gears (green) are rotating at the same speed and the loads on each output shaft are equal, they behave as one unit. Because of this the planet gears are locked as they cannot rotate.

The spinning differential divides the available torque equally between the two output shafts and, as equal traction is available at the wheels, they are both powered forward by the same amount, which can be seen if you Play the animation on the right. You may Stop the animation if required.

Cornering Action of an Open Differential

When a model car is negotiating a corner, the inside wheel has to slow down to cover a shorter distance than the outside one.

This happens automatically as the sun gears (green) meet no opposition from the planet gears (yellow) as they rotate at a different speed to each other. This movement relative to each other is effectively like that shown in the Reverse Action Animation above.

The open differential works perfectly if the car is coasted around the corner and only power applied to the wheels when the car is facing in a straight-ahead direction.

Where the open differential fails badly is when power is applied to the wheels coming out of a corner (and to a lesser extent on a bumpy surface).

This is due to the weight on both the left and right hand wheels being unequal as part of the weight of the car transfers to the outside wheel. This creates an imbalance in the load on both output axles.

The spinning differential tries to divide the available torque equally between the two output shafts, but fails because the outside wheel has higher resistance to movement than the inside wheel.

What happens can be seen when the animation to the right is Played. The planet gears lever against the outside output and transfer the available rotation to the inner output as it offers little or no resistance. You may Stop the animation if required.

As the majority of the available traction is available to the outside wheel with its greater load, the inner wheel struggles to find grip and spins. If the inside wheel is completely off the ground, there will be no drive to the car and it can only coast around the corner.

This loss of traction is commonly referred to as “diffing out” and is not limited to open differentials, other types will display the same behaviour under the right conditions.

The spinning of the inside wheel is compounded by the action of the division of the speed of the differential, the amount of rotation that would ordinarily be divided into driving both wheels equally is channelled into driving the inside wheel at twice the speed, which has an impact on the directional stability of the car when it gains traction.

Summary

Hopefully we have explained how an open differential works without creating too much confusion for you.

Many thanks to Rob Lochier of Alro Racing Systems for his technical assistance with this article.

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© 2001, 2009 by Darren Burnhill